16 Reconfigurable Antennas
open, currents travel around the slot and the antenna operates in a lower frequency band. With the diode switch closed, the effective length of the patch is shorter and the antenna operates in a higher frequency band. The slot length controls the frequency ratio between the upper and lower operating frequencies. As long as the slot length is not too long, the radiation pattern of the original antenna is largely preserved [22]. Longer slot lengths will result in radiation patterns with higher crosspolarization components. Others have since extended this concept to other microstrip structures, such as E-shaped patch antennas [23].
Although these are just some of the many examples of switched frequency reconfigurability with a range of different antenna types and geometries, they all share the common approach of discrete changes in effective length to achieve their goals.
4.2.2Variable Reactive Loading
The use of variable reactive loading has much in common with the switched reconfigurability discussed in the previous chapter. The only real difference between the two is that, in this case, the change in effective antenna length is achieved with devices or mechanisms that can take on a continuous range of values (typically capacitance) that allows smooth rather than discrete changes in the antenna’s operating frequency band.
One example of a continuously tuned microstrip patch antenna is presented in [24]. In this case, two varactor diodes (varactors) were connected between the main radiating edges of the structure and the ground plane. With a reverse bias varying between 0 and 30 V, the varactors had capacitances between 2.4 and 0.4 pF. As the bias level changed, the capacitances at the edges of the patch tuned the effective electrical length of the patch. Continuous frequency tuning over a large band is possible (20–30% as shown in [24]) depending on the antenna topology.
A one-wavelength slot antenna loaded with two one-port reactive FET components was tuned continuously in [25]. By changing the bias voltage, the reactances of the FETs were varied by changing the bias voltage, which, in turn, changed the effective length of the slot and its operating frequency. The range of tuning was about 10%, centered around 10 GHz. The patterns were essentially unchanged for this relatively small tuning range [25]. Similar tunable slot antennas equipped with varactors [e.g., 26, 27] have also been developed, which take advantage of higherorder resonances to create tunable dual-band performance. Using a transmission line model of the loaded slot resonator, the varactors’ positions can be determined to enable independent tuning of the two bands [27].
More recently, a microstrip patch antenna has been tuned using integrated RF-MEMS capacitors [28]. Shown in Figure 4.5 [28], the capacitors are implemented on a CPW tuning stub and actuated with continuous DC bias voltages up to 12 V, which produce operating frequencies
Methods for Achieving Frequency Response Reconfigurability 17
FIGURE 4.5: Frequency-tunable microstrip patch antenna with RF-MEMS capacitors and CPW tuning stub (Erdil et al. [28], ©IEEE 2007).
between 15.75 and 16.05 GHz [28]. The unique monolithic approach to the design eliminates the need for bias vias, and the resulting radiation patterns show little effect of the proximity of the tuning stub. Other microstrip antennas with slots equipped with solid-state varactors have also been demonstrated [29].
A combination of switching and reactive tuning has also been implemented to support both course and fine frequency tuning in a printed monopole antenna [30]. Based on a meander-line monopole structure, a PIN diode is implemented to provide course tuning between system bands (in this case, 2 and 5 GHz), whereas a varactor is used for fine tuning within each band [30], perhaps supporting a level of signal filtering capability at the antenna that can be used in conjunction with other radio functions.
18 Reconfigurable Antennas
FIGURE 4.6: Photograph of mechanically actuated reconfigurable antenna with movable parasitic element, providing variable operating frequency, bandwidth, and gain (Bernhard et al. [33], ©IEEE 2001).
4.2.3Structural/Mechanical Changes
Mechanical rather than electrical changes in antenna structure can deliver larger frequency shifts, whether used for switched or continuously variable bands. The main challenges with these antennas lie in the physical design of the antenna, the actuation mechanism, and the maintenance of other characteristics in the face of significant structural changes. One example of a mechanically tuned antenna was demonstrated in 1998, where a piezoelectric actuator system was used to vary the spacing between a microstrip antenna and a parasitic radiator to change the operating frequency of the antenna [31–33]. A picture of the antenna is presented in Figure 4.6 [33]. Although normally possessing a very narrow bandwidth (1%), controlled movement of the parasitic element delivered an effective bandwidth of about 9%. This example illustrates the difficulty in achieving one kind of reconfigurability without incurring changes in other antenna characteristics; the bandwidth and gain of the structure also change as a function of parasitic element spacing but cannot be individually selected [33].
Another example of continuous frequency changes enabled by mechanical changes is a magnetically actuated microstrip antenna [34]. A microstrip antenna designed for operation around 26 GHz was covered with a thin layer of magnetic material and released from the substrate. Using a micromachining process called plastic deformation assembly, application of an external DC magnetic field causes plastic deformation of the antenna at the boundary point where it is attached to the microstrip feed line, resulting in a patch positioned at an angle over the substrate. A photograph
Methods for Achieving Frequency Response Reconfigurability 19
FIGURE 4.7: Photograph of magnetically actuated reconfigurable microstrip antenna (Langer et al. [34], ©IEEE 2003).
of one prototype is given in Figure 4.7 [34]. Small changes of the angle at which the structure resides results in changes in operating frequency that preserve radiation characteristics, whereas larger angles result in frequency shifts accompanied by significant changes in the antenna’s radiation pattern. In particular, as the elevation angle between the patch and the horizontal substrate increases past 45°, the antenna’s radiation is more characteristic of a horn antenna and changes toward the pattern of a monopole antenna as the angle approaches 90°.
4.2.4Material Changes
Although changes to the conductors predominate in reconfigurable antenna designs, changes in the material characteristics of designs also promise the ability to tune antennas in frequency. In particular, an applied static electric field can be used to change the relative permittivity of a ferroelectric material, and an applied static magnetic field can be used to change the relative permeability of a ferrite. These changes in relative permittivity or permeability can then be used to change the effective electrical length of antennas, again resulting in shifts in operating frequency. As a potential bonus, their relative permittivities and permeabilities are high compared with commonly used substrate materials, translating into greatly reduced antenna sizes. Aside from any complexities resulting from the necessity of the bias structure, the main drawbacks to using standard ferroelectric and ferrite bulk materials (typically with thicknesses on the order of millimeters) are their highconductivity relative to other substrates that can severely degrade the efficiency of the antenna.
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One example of a frequency-tuned ferrite-based antenna is presented in [35], which provided a 40% continuous tuning range with the variable static magnetic field in the plane of the substrate and perpendicular to the resonant dimension of the patch. However, the radiation performance of the design left much to be desired, with cross-polarization levels that were significantly higher than those expected from a traditional rectangular microstrip antenna [35]. Others have also investigated the properties of ferrite-based microstrip antennas [e.g., 36, 37], with results indicating that factors including nonuniform bias fields and the multiple modal field distributions excited in a bulk ferrite substrate may preclude their use in practical applications.
Recently, several groups have developed ferroelectric materials in thin film form in an effort to minimize the loss introduced into the circuit while still providing a degree of tunability [e.g., 38–40]. However, most proposed applications still use tunable materials in the feed structure or in parasitic elements rather than the antenna itself due to limitations in the planar extent and achievable uniformity of the films.
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